A platinum colloid, which is a colloidal suspension comprising platinum particles dispersed in water, is known to have a reducing activity, i.e., an antioxidative activity (Patent Literatures 1 to 3 and 10). The platinum colloid has been used to produce various products such as drinks, medical products, and cosmetic products. The platinum colloid may be prepared by metal salt reduction, in which platinum ions are reduced in the presence of a colloid-protecting agent to give the platinum colloid. Production of the platinum colloid by metal salt reduction has been reported using colloid-protecting agents such as polysorbate 80 (Patent Literatures 1 to 3), glycerol fatty acid esters (Patent Literature 7), lecithin, saponin (Patent Literature 11), and ether-type nonionic surfactants (Patent Literature 13). Nevertheless, in view of the utility of platinum colloids, there is a need for a novel method for producing an improved platinum colloid that has a better antioxidative activity.